1. Field of the Invention
This invention relates to the shaping and cooling of glass sheets and particularly in the high speed production of bent glass sheets that are toughened by air quenching, and most particularly, for shaping and heat treating relatively thin glass sheets.
Shaped glass sheets are widely used as side windows in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side windows are installed. It is also important that the side windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area. During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of a glass sheet to breakage, tempering also causes a glass sheet to fracture into relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the deformation temperature of glass and into a shaping station where each glass sheet in turn is transferred onto a lifting member that lifts the glass sheet into engagement with a vacuum mold. The vacuum mold holds the shaped glass by suction while the lifting member retracts to below the substantially horizontal path. At about the same time, a transfer and tempering ring having an outline shape conforming to that of the glass sheet slightly inboard of its perimeter moves upstream into a position below the vacuum mold and above the lifting member. Release of the vacuum deposits the shaped glass sheet onto the tempering ring.
When prior art apparatus transferred a shaped glass sheet from the vacuum mold onto the transfer and tempering ring in misaligned or misoriented relation to the latter, the glass sheet shape would depart from required tolerances. No provision was available to remedy such a defect except to destroy the article so produced in order to avoid a customer complaint. In addition, while prior art apparatus kept improving the speed of glass sheet bending and tempering, there still remained a need for further improvement, particularly in the shaping and tempering of thinner glass sheets than those required prior to the need to supply thinner tempered glass windows for automobiles in order to lighten the vehicle, thereby reducing its rate of fuel consumption.
2. The Prior Art
U.S. Pat. No. 2,003,383 to Robert A. Miller and U.S. Pat. No. 2,805,520 to Lloyd V. Black are patents that show alignment stop members mounted adjacent the perimeter of a glass sheet shaping mold to limit the misalignment between the glass sheet and the mold as the sheet sags toward an upward facing shaping surface of the mold. U.S. Pat. No. 2,869,267 to William P. Bamford shows movable end stops that follow the opposite longitudinal edges of a glass sheet as the latter sags toward a mold shaping surface.
U.S. Pat. No. 3,846,104 to Seymour provides method and apparatus in which glass sheets are conveyed through a furnace on conveyor means, and heated while passing through the furnace to a temperature approaching the glass softening point. At a shaping station beyond the furnace, each glass sheet in turn is lifted by a lower outline shaping mold which raises the glass sheet into engagement with an upper vacuum mold having a shape conforming to that desired for the glass. The upper vacuum mold remains at the shaping station and holds the shaped glass thereagainst as the lower shaping mold retracts to below the level of the conveyor means. A tempering ring shaped to support the bent glass sheet adjacent its marginal or peripheral edge only, moves generally horizontally between the shaping station and a cooling station to receive each shaped glass sheet released by the vacuum mold and transfer it to the cooling station.
U.S. Pat. No. 4,092,141 to Robert G. Frank and DeWitt W. Lampman further improves the efficiency of a glass sheet bending and cooling apparatus by providing vertically movable sheet transfer means for rapidly removing from the tempering ring each bent glass sheet after the latter has had its surfaces hardened sufficiently to permit it to be conveyed on an additional downstream conveyor providing a glass sheet supporting surface at an elevation slightly higher than the level at which the glass sheet is supported by the transfer and tempering ring. However, the apparatus of this patent requires that the return movement of the tempering ring to the shaping station be delayed until after the trailing edge of the shaped, surface-hardened glass sheet has moved downstream into a position where the entire glass sheet is completely beyond the transfer and tempering ring.
The sheet transfer means for removing the glass from the tempering ring to the additional downstream conveyor according to the Frank and Lampman patent comprises a frame support and a lifting and lowering mechanism for raising and lowering the frame support and a plurality of rotating conveyor rolls having doughnut-shaped support members carried by the frame support. The apparatus is so constructed that the rotating conveyor rolls of the sheet transfer means move vertically between a recessed position clear of the movement of the tempering ring and its supporting carriage and a transfer position wherein the rotating conveyor rolls have a common upper tangential surface and provide rotating elements of a glass sheet supporting surface disposed above the level of support provided by the tempering ring and at a level of support provided by the upper tangential surface common to the rolls of the additional downstream conveyor.
The prior art patents fail to provide means at the shaping station to insure that each shaped glass sheet is oriented and aligned on the tempering ring in proper orientation and alignment thereon so that it retains its proper shape while cooling. Furthermore, the need to bend and temper thinner glass sheets (4 millimeters thick or less) than thicknesses handled previously (4.5 to 6 millimeters) makes it desirable to incorporate means to reduce the time required for the apparatus to complete a cycle of bending and tempering, particularly the time needed to transport a heated glass sheet through a shaping station and completely into a cooling station, because thin, hot glass sheets lose their shape more rapidly than thicker glass sheets and also cool more rapidly to a temperature below which it becomes difficult if not impossible to impart an adequate temper in the glass.